To make something certain.
To give an assurance that something will be done right.
To assume or take responsibility for a debt or other obligation.
The person to whom a guarantee is made.
A person who gives such a guarantee; a guarantor.
A legal assurance of something, e.g. a security for the fulfillment of an obligation.
More specifically, a written declaration that a certain product will be fit for a purpose and work correctly; a warranty
Anything that assures a certain outcome.
To feel certain about something.
To offer a bet in which one stands more to lose than the opponent; or a bet in some other way favourable to the opponent.